1. Field of the Invention
This invention relates to improvements in methods and apparatuses for dynamic information storage or retrieval, and more particularly to improvements in methods and circuitry for detection and correction of errors caused by media noise or other transition jitter, especially in information storage and retrieval systems that use a magnetic data storage medium, and still more particularly to improvements in methods and apparatuses for improving data detection in dynamic information storage or retrieval systems of the type that use post-processor data detection techniques.
2. Relevent Background
In the construction of dynamic information storage or retrieval devices, or the like, and in particular in the construction of the data channel used in digital magnetic recording systems, or the like, such as hard disk drives used in conjunction with digital computer systems, or the like, there has been significant recent interest in Partial Response Maximum-likelihood (PRML) signaling techniques. The most common PRML systems are PR4ML (a partial response class 4) and EPR4ML (extended partial response class 4). Maximum-likelihood detectors, which use a Viterbi algorithm, are generally used for these partial response channels.
In such systems, the use of EPR4 Viterbi data detection techniques is widely used. EPR4 or EFPR4 Viterbi detectors are well known, and involve probabilistic techniques for determining data states in the data channel. As data rates increase in the data channel, it becomes increasingly difficult to distinguish adjacent data pulses, and the Viterbi techniques have been found to be very useful.
Unfortunately, significant errors still occur in data detection. For example, using EPR4 techniques, a bit error rate (BER) of about 10−5 typically occurs. However it has been observed that if the signal to noise ratio in a system could be increased by, for example, only 1 dB, the bit error rate can be improved to 10−6. This represents an order of magnitude improvement. Thus, even small improvements in the signal-to-noise ratio can result in large improvements in the bit error rate using EPR4 detection techniques. This is significant since presently the requirements exist for the provision of circuits that have a bit error rate less than 10−7, and it is expected that this requirement will continue to become more stringent.
In order to enhance the BER (bit error rate) performance, recently, post-processors have been incorporated with the Viterbi detector. An example of such post-processor is that shown in U.S. patent application Ser. No. 09/229,945, filed Jan. 13, 1999, entitled POST-PROCESSOR USING A NOISE WHITENED MATCHED FILTER FOR A MASS DATA STORAGE DEVICE, OR THE LIKE, by the inventors hereof, assigned to the assignee hereof, and incorporated herein by reference. However, in recent disk drives that have a high recording density, the media noise from the magnetic disk has become an important factor that is difficult to ignore.
The media noise arises primarily from written magnetic transitions, which typically have a zigzag transition geometry. The zigzag transition geometry produces position or pulse width uncertainties or “jitter” variations of read back signals. When media noise is dominant, it is difficult for such post-processors to improve performance, since the characteristic of media noise is quite different from white noise. The spectrum of the media noise is not white, and is dependent on the data pattern written on the magnetic disk.
What is needed, therefore, is a post-processor that is effective for media noise in information storage and retrieval systems that use a magnetic data storage medium.